Hercules Globular Cluster (M13, NGC 6205)

The Hercules Globular Cluster (M13, NGC 6205) is the most prominent globular cluster in
the northern half of the sky. It lies in the constellation Hercules and was discovered by Edmond Halley in
1714, who noted that "it shows itself to the naked eye when the sky is serene
and the Moon absent."

In small telescopes, M13 appears as a misty patch, denser toward the center;
in large scopes it's revealed as a nearly perfect sphere of stars. Although
it spans in total well over 150 light-years, the bulk of its more than one
million stars are concentrated into a core region whose diameter is under
100 light-years. In this inner domain the average distance between stars
is only about 1 light-year compared with a typical gap between stars in
the solar neighborhood of just over 3 light-years. M13 was selected in 1974,
by Frank Drake and Carl Sagan, as a target for one of the first radio messages
addressed to possible extraterrestrial races, sent by the Arecibo Telescope
(see Arecibo Message)